1. Field of the Invention
The invention relates to fluid barrier seals that maintain a vapor barrier between relatively movable parts. An exemplary application is a seal between a vessel wall and a substantially vertical impeller shaft of a rotational mixing apparatus.
2. Prior Art
An element that traverses the wall of a closed vessel can have a vapor tight seal between the element and the vessel, even though one may be stationary while the other is movable. For this purpose, a quantity of liquid can be arranged to occupy the space between the relatively movable parts, thereby providing a hydrostatic or liquid barrier separating the vapor in the vessel from the ambient atmosphere. The liquid flows as needed to conform to the space between the parts. There are various possible applications for such a seal. In this disclosure, a liquid barrier seal is applied to sealing between a mixer vessel or the like and a vertical impeller shaft that traverses the wall of the vessel. The invention is applicable to other similar applications as well.
Such a seal can be made to withstand a difference in pressure between the inside of the vessel and the ambient air outside. In one arrangement, a downwardly opening cup-shaped cylinder is attached and sealed to the part traversing the vessel wall, for example a rotating vertical impeller shaft. The downwardly opening cup-shaped cylinder extends axially downward, and axially overlaps an upwardly opening annular cylinder that is attached and sealed to the vessel. The annular cylinder contains a barrier fluid that seeks its lowest level due to gravity, residing in the bottom of annular space provided. The downwardly opening cup-shaped cylinder is positioned with its lip extending downward into the annular cylinder, below the surface of the barrier fluid in the annular cylinder. The downwardly opening cup-shaped cylinder (with the impeller) and the upwardly open annular cylinder (on the vessel) are relatively rotatable. A vapor seal is maintained by the barrier fluid, which occupies the spaces between the axially coextensive and radially interleaved walls of the annular cylinder and the downwardly opening cup-shaped cylinder, respectively.
Differential pressure between the vessel and the ambient causes the level of the barrier fluid to differ between the inside and outside of the cup-shaped cylinder, in the manner of a manometer or pressure gauge, hence the name xe2x80x9cmanometricxe2x80x9d seal. The difference in height of the fluid levels, and the maximum difference permitted by the extent of cup/cylinder submergence determines maximum pressure difference between ambient and closed vessel. In a static sense, the pressure difference must present a force that is less than the force that could displace the fluid from the space between the relatively movable parts. In a dynamic sense the matter is influenced by the rate of relative rotation, which induces turbulence.
The foregoing subject matter is discussed herein with respect to the example of a vertical shaft in a vessel, wherein a vertically oriented cup-shaped cylinder fits into an annular cylinder, so as to provide radially interleaved, axially overlapping walls. The axis need not be precisely vertical for such a seal to function. The same considerations apply, for example, to arrangements having inclined axes wherein there is a vertical component over which cup-shaped and annular cylinders or similar structures may overlap over a span that is occupied by the barrier fluid and has at least some vertical extension.
Manometric seals are most applicable as vapor/gas seals in relatively slow rotating mixing equipment because the extent to which the liquid barrier can bear a pressure difference is not complicated by turbulence. The basic components of a typical seal are shown in FIG. 1, labeled xe2x80x9cPrior Art.xe2x80x9d A downwardly opening cup-shaped cylinder 1 is connected and sealed to a rotating shaft 2. Thus the cup-shaped cylinder rotates with the shaft. A stationary upwardly-opening cylinder or annular container 3 contains a barrier fluid 4 that seeks its lowest level due to gravity. If there is a difference in pressure within and without of the vessel, the difference appears as a difference in the fluid level in the annular container 3 on either side of the downwardly opening cup-shaped cylinder 1. The pressure difference produces a fluid head.
Referring to FIG. 1, vessel internal pressure bears on barrier liquid surface 6 inside of the downwardly opening cup-shaped cylinder 1. Ambient pressure acts on barrier fluid surface 5 on the outside of cup-shaped cylinder 1. The cup-shaped cylinder rotates with the shaft 2, whereas the annular container 3 is stationary.
When the shaft is not rotating or at low rotating speeds, the liquid surface is undisturbed. The inside/outside pressure differential can vary, moving the positions of inside and outside fluid surfaces 5, 6 so that one or the other is higher or lower. The seal fails (leaks) in that case if the pressure differential becomes sufficient to force the level of fluid surfaces 5 or 6 all the way down to the level of the bottom lip of the downwardly opening cup-shaped cylinder.
With rotation of the cup-shaped cylinder 1, fluid adjacent to the surface of the cup-shaped cylinder tends to be carried along with the rotating cylinder by friction, while fluid adjacent to the stationary surfaces of the annular cylinder is retarded by friction. The shear is such that surfaces 5, 6 can become turbulent. Vortices 9 can be generated, resulting in the formation of discrete bubbles that are spread throughout the barrier fluid volume. Any bubbles 10 that are formed on one radial side or the cup-shaped rotating cylinder 10 and migrate to the other side, may be carried to the surface by their buoyancy, where their vapor contents is released. Gas (vapor) that is released by such bubbles is a form of leakage passing the fluid barrier seal.
In view of the possibility of leakage as described, for proper seal operation the cylinder tangential speed should not exceed a limit of about 4.5 m/s (900 ft/min) for a barrier fluid having the viscosity characteristics of water. At or near this limit, it is appropriate to provide a series of axially extending baffles 11 protruding radially inwardly from the outer stationary wall of the annular cylinder. At even higher rotating speeds and/or in the absence of the circumferential obstruction presented by such baffles, substantially all the barrier fluid can be put into rotation together with the downwardly opening cup-shaped cylinder, leading to turbulence that disrupts seal operation completely.
The differential pressure limit for a manometric seal depends on the density of the barrier liquid and the dimensions of the seal, such as axial height of the seal. These aspects determine the mass of fluid displacement at the maximum fluid level difference on the radially opposite sides of the downwardly opening cup-shaped cylinder. For practical reasons the height of a seal as described seldom exceeds 1 m (3 ft).
The barrier fluid must be compatible with any process media that will contact the barrier fluid. The barrier fluid preferably has a low viscosity to limit power consumption. In many applications water is a possible choice, but for water and other often-volatile low viscosity liquids, evaporation may be a drawback.
It would be advantageous to increase the possible tangential velocity range of a manometric seal. It would also be advantageous to do so in a seal that can employ low viscosity barrier fluid, while controlling the problem of evaporation.
According to an aspect of the present invention, it has been found that the useful range of tangential cylinder speed of a manometric seal can be increased to over 10 m/s (2000 ft/min) by use of a combination of vertical and horizontal baffles on one or preferably both of the surfaces of the stationary annular cylinder that face the cup-shaped rotation cylinder of the seal. The vertical baffles extend axially and radially to provide obstructions in a circumferential direction. These baffles prevent the barrier fluid from rotating, and reduce the formation of vortices that could disrupt seal operation. Furthermore, horizontal baffles extend circumferentially and radially, and reduce the vertical mobility and mixing efficiency of the barrier fluid. The horizontal baffles greatly reduce the number of small gas bubbles that reach the bottom lip of the downwardly opening cup-shaped cylinder and potentially pass under the seal bottom and result in leakage.
Shaft work and frictional heating of the barrier fluid dissipate the energy that is released in a rotating seal element. It is an aspect of the invention to optimize the structure of the container for the barrier fluid so as to reduce mixing, turbulence and consequent gas bubble generation. Full scale testing has suggested that energy dissipation in a high-speed seal as described should be keep below 6 W/kg (30 Hp/1000 gal).
It is another aspect of invention to eliminate evaporation losses so that the seal is substantially maintenance free and it is generally not necessary to xe2x80x9ctop offxe2x80x9d the barrier fluid level in the seal. This is accomplished in one embodiment by using process liquid as a barrier fluid. If conditions allow, a process fluid""s vapor is condensed and returned to the seal. Excess barrier fluid is put back in to vessel through U-pipe liquid locks.